We have detected a Heavy chain antigen (183-104) common to all IgM myeloma proteins of BALB/c mice. However, 183-104 is present on only a small proportion (less than or equal to 10%) of serum IgM molecules in norma BALB/c mice. As 183-104 is located in the Heavy chain constant region (C micron), we conclude that IgM myeloma proteins represent a subclass of IgM. Experiments in progress are designed to answer whether sugar or amino acid residues are responsible for 183-104 determinants.